Apparatus for initiating fractures in earth formations



Aug. 20, 1963 B. GILBERT 3,101,051

APPARATUS FOR INITIATING FRACTURES IN EARTH FORMATIONS Filed July 31,1957 I 4 Sheets-Sheet 1 m 01 4 Q1 N Aug. 20, 1963 B. GILBERT 3,10 ,05

APPARATUS FOR INITIATING FRACTURES IN EARTH FORMATIONS Filed July 31,1957 4 Sheets-Sheet 2 I .1 llllll 42 ml l l lllllffl FIG.3

B. GILBERT 3,101,051

APPARATUS FOR INITIATING FRACTURES IN EARTH FORMATIONS 4 Sheets-$heet 3Aug. 20, 1963 Filed July 51, 1957 Aug. 20, 1963 r B. GILBERT 3,101,051

APPARATUS FOR INITIATING FRACTURES IN EARTH FORMATIONS Filed July 51,1957 4 Sheets-Sheet 4 United States Patent 3,101,051 APPARATUS FORINITlATlNG FRACTURES EN EARTH FURMATIONS Bruce Gilbert, Dallas, Tex.,assignor to The Western Company of North America, a corporation ofDelaware Filed July 31, 1957, Ser. No. 675,424 6 Claims. (Cl. fill-2d)This invention relates broadly to apparatus for initiating or producingfractures in earth formations and particularly to apparatus for creatingfractures having a predetermined orientation in such formations. Stillmore particularly, the invention relates to apparatus for initiatingfractures at a predetermined depth and in la redetermined orientation inan earth formation penetrated by a well bore hole.

It is frequently desired to create and extend fractures in earthformations to facilitate mining or quarrying operations or the recoveryof underground fluids such as petroleum, natural gas or water. Thepurpose of such fractures in mining and quarrying operations isgenerally to separate masses of rock or mineral from large formations.In petroleum, gas or water recovery operations, the ultimate purpose issimilar, ire. to produce a fracture or separation in a rock or mineralformation, but the techniques employed are somewhat different. Inpetroleum production, for example, it is common practice after drillinga well bore hole into an oil bearing earth formation to attempt toincrease the permeability of the formation by creating thereinartificial fractures which facilitate the flow of petroleum from theformation into the well. For many years such fractures were produced bydetonating massive charges of nitroglycerine in the Well adjacent to thedesired formation. This method has been superseded in recent years byhydraulic fracturing techniques, such as those described in US. ReissueNo. 23,733. In hydraulic fracturing methods a viscous liquid, eitherhydrocarbon or water base, is injected into the case well undersufficient hydraulic pressure to cause the exposed formation at thebottom of the well to break down thus creating and extending fractureswhich increases the permeability of the oil bearing stratum. Send orother granular material is commonly incorporated in the fracturing fluidas a propping agent to prevent closore of the newly created drainagechannels by the pressure of the overburden of earth. The fluids used inhydraulic fracturing are generally viscous liquids containing chemicalagents which cause the viscosity of the. liquid to decrease after aperiod of time so that the fracturing fluids can be flushed from thenewly created fractures either by fluid produced from the formation orby fluid injected into the formation for that purpose. This fracturingprocedure generally causes a significant increase in the production ofthe treated well.

A hydraulic fracturing operation ordinarily produces only a singlefracture since it is usually impossible to pump sufficient fracturingfluid to raise the treatment pressure enough to create a second fracturewhile the initial fracture is making fluid. For this reason techniqueshave been developed for isolating and treating short sections of a wellbore individually in order to produce fractures at different levels inthe well. straddle packers, for example, have come into common use forthis purpose making it possible for the operator to control the locationat which an artificial fracture is initiated in an underground earthformation. It has long been recognized, however, that it would be highlydesirable not only to control the 4 hydraulic fracturing operation so asto produce a single fracture at the exact depth in the well at which oilhas been determined to be present but, in addition, to orient the planeof the fracture so that it falls entirely within the producingformation. Positive control of both the depth and orientation at whichan artificial fracture is created would of course make it possible tocreate and extend only those fractures which would be most likely toprovide the greatest increase in the useful production of a well. Theutility of a fracture initiation apparatus providing such positivecontrol is apparent. For example, a more or less horizontal petroleumproducing stratum is often located close to a similarly disposed Waterproducing stratum. When a well drilled through formations of this typeis fractured by conventional methods, it frequently happens that therandomly created fracture is oriented in a more or less vertical planecutting both the petroleum and water producing strata and consequentlyflooding the well with water. It is obvious that if it were possible tocreate a [generally horizontal fracture at the desired depth in such awell adjacent to the petroleum bearing stratum there would be nocommunication with the adjacent water bearing stratum and con sequentlythe hydraulic fracturing treatment would produce a valuable oil wellrather than a worthless water well.

The apparatus of the present invention provides the desired positivecontrol of the plane of orientation of a fracture created in an earthformation. This apparatus may be employed according to the methoddisclosed in my copending application for United States Letters Patent,Serial No. 700,144, filed December 2, 1957, now Patent No. 3,05 8,521entitled, Novel Method of Initiating Fractures in Earth Formations.

It is an object of the present invention to provide novel apparatus forcreating a fracture having a preetermined orientation in an exposedearth formation such as the wall of a quarry or underground tunnel.

It is a particular object of the invention to provide apparatus fortreating an exposed earth formation in a well bore hole to establish theplane of any fracture created or extended by a subsequent hydraulicfracturing operation.

It is a particular object of the present invention to to provide novelapparatus for initiating a fracture in an earth formation penetrated bya well bore hole at a predetermine depth in the earth and in apredetermined orientation with respect to the bore hole.

It is a further particular object of the invention to provide anapparatus for the creation or initiation of a generally horizontalfracture in an earth formation, i.e., a fracture in a plane more or lessparallel with the horizon, or in different planes, such as one about 45from a plane parallel with the horizon.

It is another object of the invention to provide an apparatus containinga plurality of shaped explosive jet charges which, when simultaneouslydetonated, produce a plurality of elongated cavities critically spacedin a row lying in a predetermined plane in an adjacent earth formation.

It is an additional object of the invention to provide a fractureinitiation apparatus which i ru ggedly constructed to withstand use inthe field by semiskilled Workers and which is capable of being run downa well casing to a predetermined depth at high speeds to the timerequired-to treat a well.

It is a further object of the invention to provide an inexpensive,easily assembled, expendable fracture initiation apparatus.

These and other objects of the invention are attained by providing afracture initiation apparatus containing a plurality of shaped explosivejet charges arranged in at least one straight row of at least two jetcharges and means for detonating the charges simultaneously; thestraight row or rows of shaped charges being permanently or adj-usta-blyoriented in a predetermined plane. Apparatus according to the inventionfor the creation of fractures in earth formations penetrated by a wellbore hole is designed and constructed to be lowered into ordinary wellcasing at high speeds. Individual shaped explosive jet charges of thenovel apparatus are designed to penetrate the charge container, Wellcasing, surrounding cement sheath and the adjacent earth formation toproduce an elongated generally cylindrical or conical cavity in thelatter; The plurality of' shaped explosive charges in the fractureinitiation apparatus are spaced apart or shielded to prevent interchargeinterference. when the charges are detonated simultaneously. The jetcharges are arranged to pro-duce a plurality of cavities in a singleplane in the adjacent earth formation in a spatial arrangement whichestablishes the orientation of the plane of the resulting fracture.'Fract-ures created by the apparatus of the present invention may beextended by conventional hydraulic fracturing techniques.

I Additional objects and advantages of the invention.

will be apparent from the following detailed description. of typicalembodiments thereof taken in conjunction with the accompanying drawingsin which:

. FIG. 1; is a diagrammatic sectional view of an earth formationpenetrated by a cased well bore hole containing a fracture initiationapparatus of the present invention suspended on an electricallyconductive cable;

. FIG. 2 is an exploded perspective view partially in section of thefracture initiation apparatus of FIG. 1;

FIG. 3 is a side elevational view of a shaped charge container whichforms, a part of the apparatus of FIG. 1;

FIG. 4 is a cross sectional view partially broken awayof the chargecontainer of FIG. 3 taken along the line 4-4 showing six shaped chargestherein;

5 is an elevational view in section of a second embodiment of thefracture initiation apparatus of the present invention;

FIG. 6 is a cross sectional view of a shaped charge container whichforms a, part of the fracture initiation ppa atus. ofFIG. 5;

FIG. 7 is a cross sectional view of the apparatus of FIG. 5 taken alongthe line -'l--7; and p FIG. 8 is an elevationalview in section of theapparatus of FIG. 5 showing additional details.

FIG. 1 shows a typical well which be an oil, gas or water injection orproduction well in actual practice although described herein as. an oilproduction Well for purposes of illustration. An earth formation 11 isshown penetrated by a well bore hole 12 containing well casing 13anchored-at the. bottom of the well by means of a sheath ofcement 14. Aconventional well head 15 is attached to the upper end of the casing 13which extends above ground level 1 6. A typical fracture initiationapparatus 17 of the present invention is shown lowered into the well toa predetermined depth adjacent to an oil hearingstratum 18 of earthformation 11. Fracture initiation apparatus 17 is connected to the lowerend of a heavy metal sinker bar .19 attached to cable head 2th ofconductor cable. 21. The conductor cable 21 is suitably a steel wirecable, containing insulated electrically conductive core, of the type incommon use for lowering well servicing equipment into oil wells and fortransmitting electrical impulses tosuch equipment. The electricalconductor incable 21.1is electrically connected to the fractureinitiation apparatus. 17 through cable head 2%) and a conductor passingthrough sinker bar 19. When it is desired to produce a fracture in theoil bearing stratum 18, an electrical impulsefrom a source on thesurface is passed down the conductor cable -21 to detonate fractureinitiation apparatus 17 causing penetration of the well casing 13,cement sheath 14 and the surrounding earth formation to produce thereina fracture having a predetermined orientation within the plane of theoil bearing stratum as will be explained in detail below. The fracturethus initiated 4 can then be extended by known hydraulic fracturingtechni ues.

FIG. 2 is an exploded view of fracture initiation apparatus 17 of FIG. 1showing its component parts and their relationship to the apparatus as awhole. A detouator housing 22 is threaded externally at each end;threads 23 serving to engage the sinker bar 19 as shown in FIG. 1 andthreads 24 serving to engage a one-to-three adapter 25. The detonatorhousing 22 is bored axially throughout its length to provide a detonatorchamber 26 and a booster charge chamber 27. A detonator 28 which maysuitably be an electrical blasting cap, provided with positive andnegative electrical leads 29 and 30 respectively, is shown in detonatorchamber 26. A plurality of booster charges 31; which may suitably belightly compressed RDX explosive, are shown in chamber 27; each boostercharge being crimped to one of a plurality of equal lengths of explosivefuse 32 which may suitably be Primacord fuse containing PETN(pentaerythrityl tetranitrate) or RDX (cyclotrirnethylenetrinitramine).explosive. Adapter 25 contains an axially bored well 33 threadedinternally to engage threads 24 of detonator housing 22. Well 33'communicates with a plurality of fuse passages 34 running through-theadapter papalllel to its axis and tel minating in ports 35-. A portionof each of the fuse passages 34 adjacent to ports 35 is threadedinternally to receive and engage an externally threaded nipple 36 of ahigh pressure fitting. A plurality of fuse tubes 37 fitted withcompression fittings and threaded at each end are; connected to adapter25, one fuse tube being associated with each fuse passage 34, by seatingthe end :of the tube against the end of an externally threaded nipple 36screwed into port 35 and tightening a nut 38 over threads onto thenipple to form a seal. :Fuse tubes 37- each contain a pair of olfsets 39to provide passages of equal length for the equal lengths of fuse 32. Acylindrical jet charge container 40 is provided with a plurality of fusepassages.

(mown inFIG. 3) originating from ports. 41 the centers of which portsare spaced along a diameter of the upper flat surface of the chargecontainer. A portion of each of the fuse passages adjacent to the ports41 is threaded to engage nipples 36' which are connected -to the ends offuse tubes 37 by means of nuts 38 over compression fittings in the samemanner as the fuse tubes are connected to the adapter 25. The maincharge container 40 is PTO!- vided with two circumferential grooves 42;one groove, being adjacent and parallel to, but slightly spaced from,each end of the cylindrical container. The grooves 42 in container 40are provided with O ring gaskets 43.,

A hollow cylindrical sleeve 44 surrounds the; container 40 between thegaskets 43 A curved \guide 45 is press fitted over each end of thecontainer 40* against an O ring gasloet 43 and the edge of the s] ve 44.The plurality (three in this case) of equal lengths of explosive fuse 32crimped to the booster charges. 31 emerge from booster charge chamber27, enter well 33 of adapter 25 a group and then pass individuallythrough fuse passages 34,

fuse tubes 37 and ports 41- into the main charge container 40 whereinthey serve to detonate charges of the fracture initiation apparatus.Inasmuch as the explosive fuse 32 employed in the apparatusof thepresent invention .detonatesat a constant rateperunit of length, the useof equal lengths of fuse initiated by a common detonator 28 insuressimultaneous detonation of the jet charges in container 40. Mechanicaljoints ofvarious types can be substituted for those shown and dew.scribed in the apparatus of FIG. 2. It is essentialhowever, that alljoints be liquid tight if the apparatus is to.

come in contact with liquids which would desensitize or otherwiseinterfere with the proper functioning of the explosive charges or fuses.

FIG. 3 is a side elevational viewof the. shaped charge container 40 ofthe apparatus illustrated in- FIG. 2. Circumferential grooves 42 areshown minus 0 ring gas lcets 43*. The ports 41 shown in FIG. 2 open intointer the main explosive) nally threaded sockets 46 which are adapted toreceive the nipples 36' of FIG. 2. Fuse passages 47 communicate withsockets 46. Six shaped charge chambers 48, of which three are visible inFIG. 3, are provided in change container 40. A portion of the metallicweb between adfaced to fire in one direction while those in the otherrow are faced to fire in the opposite direction. One of the three fusepassages 47 is disposed between the bases of each pair of opposed shapedchanges 50 so that a single fuse can detonate two charges. While theshaped explosive charges are shown in a fixed position, they can beinstalled with means for making them adjustable, as will be apparent tothose skilled in the art.

The individual jet charges 50 are composed of a metal cone 51, a shapedmass of explosive 52. packed around the exterior surface of the cone inthe form of a cylinder having a convex base, a concave heavy metal base53 containing a centrally located ignition opening 54, an explosivebooster charge 55 positioned in opening 54, a metal foil booster chargewrapper 56 surrounding the booster charge on all sides except that incontact with the main charge 52, and a metal foil shaped charge wrapper57 encasing the entire shaped charge 50. The shaped charges 50 shown inthe apparatus of FIG. 4 are similar to the jet charges currentlyemployed in the perforation of well casing although modified in severalimportant respects due to the space limitations imposed upon thefracture initiation apparatus by the internal dimensions of conventionalwell casing. In apparatus of the present invention for use in cased wellbore holes it is desirable to employ short-coupled shaped charges, ize.,those in which the diameter of the metal cone 51 or face of the chargeis maximized while the overall length of the charge is minimized. Theexplosive mass of conventional jet charges is more or less conical inshape whereas in the short'coupled shaped changes shown in FIG. 4 thepoint of the cone of explosive has been eliminated resulting in a bluntbased charge which would not function properly if it were not for theaddition of the dense metal base 53. Such changes must be initiated atthe concentrically located booster charge in opening 54 in order tofunction properly. The dense metal base also serves to protect theexplosive charge from premature initiation by the explosive fuse whichlies across the base of the charge.

A preferred embodiment of the fracture initiation apparatus illustratedin FIGS. 1, 2, 3 and 4, which is designed to be lowered into /2" outsidediameter well casing at high speeds, has a diameter of 4% at the sleeve4-4. Preferred short coupled shaped charges for use in this apparatushave a Zinc base 53 and aluminum foil wrappers 56 and 57 and are 1" indiameter and about 1%" long. The charges are spaced 1.155".apart betweencenters in the arrangement shown inFIG. 4.

Thus, with the shaped charges being 1 inch in diameter and spaced 1.155inches apart between centers, the space between the charges is 0.155inch and the ratio of charge diameter to distance between the charges is6.45. The air chambers in such an apparatus are 0.155" wide and 0.375deep and extend inward from the peripheryof the charge container to thepoint where the explosive mass of the adjacent shaped charge firstcontacts the heavy metal base of the shaped charge. It has been foundthat the removal of sufficient metal web to form air chambers of thesedimensions between adjacent charges causes the jets from the outsideshaped charges in a row of three to converge at an angle of about 12when adjacent charges in the apparatusare arranged with their axesparallel.

When a row of three 1" diameter shaped charges spaced 1.155" betweencenters is fired it produces a row of cavities in the target materialhaving a ratio of cavity diameter to web thickness between cavitiesvarying from about 0.25 to 1.0. This is within the operable range ofratios of cavity diameter to web thickness Which is necessary to achievecontrolled fracture initiation in the majority of earth formations withapparatus of the present invention. An operable range of ratios ofcavity diameter to web thickness produced by the apparatus of theinvention comprises at least 0.25 up to, but not including, infinity,such as 10 00 or 100. The desired ratio will depend somewhat upon thenature of the target material. A ratio of infinity would produce acontinuous opening and at this value no web material would remain toseparate the holes in which stress would be produced. At a value of lessthan 0.25 the stress concentration produced in the web is infinitesimaland the value of the perforation is'largely lost. The preferred ratio isbetween 0.75 and 1.0.

The jet charges employed in the apparatus of the present invention whenfired into a steel target at stand-off (air space) from the targetproduce a perforation or conical cavity through three inches of steel;the diameter of the cavity tapering from about /2" at entry to about A3at the exit. This tapering of the cavity is also obtained in othertarget materials and since it is necessary as noted above to maintain aratio of cavity diameter to web thickness within the range from 0.25 toless than infinity it is important to offset the variation in the ratiocaused by the tapering of the cavities. convergence of the outside jetsfrom a row of three adjacent shaped charges spaced as noted :aboveserves this purpose since as the tapered cavities become smaller theouter cavities converge maintaining a relatively constant ratio ofcavity diameter to web thickness between the cavities.

As noted above it is essential for proper operation of the fractureinitiation apparatus that all shaped charges therein be detonatedsimultaneously. This can be accomplished by providing equal lengths ofdetonating or explosive fuse having a constant rate of burning per unitof length. :These equal lengths of fuse as noted above in thedescription of FIG. 2 are crimped to booster charges 31 in chamber 27 incontact with the detonator 28. Initiation of the detonator or blastingcap 28 sets off the booster charges 31 simultaneously initiating theequal lengths of fuse. The separate lengths of fuse burn at the samerate thus initiating the booster charges and detonating shaped charges50 simultaneously. Without simultaneous detonation of the charges thenecessary stresses do not result between the holes in the earthformation.

FIG. 5 is an elevational view in section of a second and preferredembodiment of the present invention which is designed for use in a casedwell. Contact sub 60, which like sinker bar 19, serves both to connectthe conductor cable to the fracture initiation apparatus and as a weightto facilitate lowering the apparatus into a well, is provided with asocket 61 adapted to receive conductor cable head 62, a socket 63adapted to engage a fracture initiation gun assembly, and a chamber 64providing communication through the contact sub between the two sockets.Chamber 64 contains a conductive core 65 surrounded by an insulator 66which serves to hold the core firmly in place so that the uninsulatedend thereof protrudes into socket 61 where it makes electrical contactwith the internal insulated conductor (not shown) of cable head 62. Adetonator assembly consisting of insu lator plug 67, two electricalleads 68 and 69 and a blasting cap 70 is held in place in the lower partof chamber 64 by means of plug 67. Electrical leads 68 and 69 are eachconnected to blasting cap 70 at one end and run The 12 7 throughinsulator plug 67 without contacting each other. The blasting cap 70 maybe any of the commercially available high temperature (stable at welltemperatures) ibla'sting caps used in oil well operations, such as DuPont E-2B. EB. caps. The other end of lead 68 is formed into a coilspring which contacts the conductive core 65 of the contact sub. Theother end of lead 69 is pinched between insulator plug 67 and the wallof chamber 64 of the contact sub. The electrical circuit is completedthrough the body of the contact sub 60 which is in contact 'with theouter conductor of the cable head 62.

The fracture initiation gun assembly comprises a detonator housing 71and a charge container consisting of a sleeve 72 and a bottom plate 73.Detonator housing 7.1 is adapted to. engage socket 63 of the contact sub60. Shoulder 74 of the housing 71 contains a circumferential groove 75adapted to receive an O ring gasket 76 which forms a seal. betweenshoulder 74 and seat 77 of socket 63. The detonator housing 71 containsa concentric bore 78, large enough to receive blasting cap 70* which issuspended from electr-ical leads 68 and 69 imbedded in plug 6-7 in thecontact sub 60. The bore 78 opens into a booster charge chamber 79.containing three-to-one adapter 80 which consists of a cylindrical metalcase partially closed at one end only and having an initiation port 81in the center ofthat partially closed end, the three-to-one adapter 80'contains four booster charges '82, one protruding through the initiatorport 81 to contact blasting cap 70 in bore 78 and the others disposedaround the centrally positioned booster charge. Each of the three outerbooster charges 82 has. one of a plurality of equal lengths of explosivefuse 83 crimped thereto.

Socket 84 of sleeve 72 of the main charge container is adapted toreceive and engage the detonator housing 71, a. seal being provided by Oring gasket 76 in groove 85 circumscribed around socket 84 in the top ofsleeve 73. The charge container is completed by means of bottom plate73, flange 86 of which is sized to be press fitted into the open end ofsleeve 72. A liquid tight seal be tween the sleeve and bottom plate-isprovided by O ring gasket 76" in circumferential groove 87 in flange $6.A plurality of equally spaced slots 88 are provided in the skirt 89 ofbottom plate 73. The slots 88 serve to aid in causingthe fragmentation.of skirt 89. A plurality of shaped explosive charges 90 are shown in thecharge block 91 within the charge container composed of sleeve 72 andbottom plate 73. a

FIG. 6 is a cross sectional view of the apparatus of FIG. taken alongthe line 66 showing the circular cross section of sleeve 72 of thecharge container, two charge blocks 91 disposed back to back along adiameter of the sleeve 72 and; shaped explosive charges 90 disposed inbore holes 92 of the charge blocks 91. Each charge block 91 containsthree bore holes 92, arranged in a straight row, the axis of the centralbore hole being perpendicular to the back of the. charge block and adiameter of the circular cross section of sleeve 72. The axes of the twooutside bore holes in the charge block make an; angle of 8 with the,axis of the central bore hole. The, shaped charges 90; are inserted; inthe bore holes 92 of. the, charge blocks 91 to form two opposed rows ofchargeswas described-above'in theapparatus of FIG; 4.

The shaped charges 90 employed in the preferred apparatus of FIG. 5 aresomewhat difierentvfromrthose employedin the apparatus of FIG. 2; Thesejet charges are short; coupled: as are the shaped charges 50 of FIG. 4since both are designed for fracture initiation apparatus useful, in.well bore holes. Charges 90, however, differ from charges. 50 in that.they are provided. with a heavy dense, metal case 93, suitably of zinc,which surrounds the shaped charge on all sides except thev face of thecharge. The great density of the zinc case prevents absorption of theshockwfromthe adjacent charge and per mits. the tips of the charges tobealmost in contact with each, other since the amount of explosive atthe tips is small. The heavy metal case contains an ignition opening- 94in the center of its base, a booster charge 95 in the ignition opening,a main explosive charge 96, and a metal cone 97, suitably a 60 coppercone, similar to those employed in the shaped charges 50 describedabove. The explosive fuse 83 is shown in contact with the boostercharges 95 of opposed pairs of shaped charges 90. Air standoff spaces 98between the shaped charges and the sleeve 72 of the charge container arenecessary for proper functioning of the charges.

FIG. 7, which is a cross sectional view of the apparatus of FIG. 5 takenalong the line 7-7, shows the circular cross sections of the detonatorhousing 71, three-to-one adapter 80, and booster charge chamber 79, andthe arrangement of the four booster charges 82 within the latter.

FIG. 8 is an elevational view in section of the apparatus of FIG. 5taken at a angle with respect to FIG. 6. The configuration of chargeblocks 91 is clearly shown by a comparison of the end view of saidblocks in FIG. 8 with the side view of FIG. 5. In FIG. 5 the chargeblocks are shown to be co-extensive with the cross section of the voidspace of the charge container consisting of sleeve 72 and bottom plate73 whereas in FIG. 8 it is seen that the blocks 91 occupy only thecentral portion of the charge container leaving air space for sufiicientstand off to insure proper functioning of the charges. I

As noted above fracture initiation apparatus of the 7 present invent-ionconsists essentially of at least one straight row of at least twocritically spaced jet charges and means for detonating the chargessimultaneously. Apparatus of the invention must be adapted to produce atleast two and preferably three, more or less parallel elongated cavitiesspaced along a straight line on the. face of the earth formation to befractured since it is the plane defined by such cavities whichdetermines the plane of the.

fracture. Apparatus having from two to five shaped charges per row issuitable for use in well bore holes although three charges per row isprefered since this number is sufficient to define the desired plane'ofthe fracture with. certainty and yet can be fitted within the narrowconfines of an apparatus capable of being lowered into a conventionalwell casing. Of course. any number of charges per row from two on up maybe employed in apparatus where there is no limitation as to size.Additional control of the plane of fracture is afforded in appar-atusfor use in a well bore hole by employing two rows of charges faced inopposite directions in the same plane. When the'two opposed rows ofcharges are detonated, rows of cavities are produced in opposite sidesof the bore hole thus creating fractures which when extended by knownhydraulic fracturing methods may meet to form a single fractureradiating in all directions 360 around the bore hole.

The spacing of individual shaped charges in a row of charges in theapparatus is an important feature. of the invention. The chargesrmay bespaced as close together as possible, i.e., to the point where adjacentcharges are in direct contact,'so long as the charges can be firedwithout intercharge interference. The greatest spacing which ispermissible between adjacent charges in a row is, therefore, the mostsignificant limitation. Although this distance varies depending upon thestrength, homogeneity and character of the earth formation to befractured it is possibleto set practical limits for the majority offormations. ilt has been found that apparatus of general utility must becapable of producing a row of cavities in which the ratio of cavitydiameter, conveniently measured at the entrance, to the thickness of theweb between adjacent cavities lies in the range from 0.25 to less thaninfinity. Apparatus of; the present invention using shaped charges ofthe character described will produce a row of cavities having this ratioof cavity diameter to web thickness.

It is necessary that all jet charges be detonated simultaneously sincepremature. detonation of one charge ina row normally causes sideinitiation of the adjacent charge. This results in a malformed jet whichfails to produce the required cavity. If it were possible, however, toprevent side initiation of adjacent jet charges confined in the requiredspacing it would be unnecessary to detonate the charges simultaneously.lntercharge interference is avoided in jet charges 50 of FIG. 4 byproviding air spaces 49 between adjacent charges. The metal eliminatedto make air chambers 49 is removed to prevent transmission of sufficientshock to cause intercharge interference. The problem of interchargeinterference is solved in jet charges 9d of 'FIG. 6 by the use of aheavy metal case around the charge.

The apparatus of the present invention is designed to be operated bysemiskilled personnel who need not understand the internal workings ofthe mechanism in order to achieve the desired results. Apparatus for usein mining or quarrying operations is placed adjacent to the face of thequarry or other formation either by suspension on a cable or byattaching the apparatus directly to the formation by any suitable means.The apparatus is then detonated by energization of the electricconductor cable after clearing the area of personnel. The detonation ofthe shaped charges produces a plurality of elongated conical cavitiesand a communicating fracture in a predetermined orientation in thetarget formation and in most cases completely destroys the fractureinitiation apparatus. Fracture initiation apparatus designed for use inwell bore holes can be lowered into the well on a conductor cableunwound from a drum mounted on a suitably equipped truck. The fractureinitiation apparatus is positioned at the desired depth by measuring thelength of cable used. Additional confirmation of the position of theapparatus in a cased well can be obtained by means of an electroniccollar counter. This device registers on an indicator at the surfacewhen the fracture initiation apparatus passes the juncture betweenlengths of well casing. Inasmuch as the length of a section of casingand the number of sections in the well is usually known, the apparatuscan be positioned accurately at any desired depth by measuring from thenearest casing collar the exact depth of which can be calculated. Whenthe apparatus has been placed at the desired depth in the well it isdetonated by energizing the conductor cable. The incipient fracturecreated in the formation at the bottom of the well is then extended by ahydraulic fracturing operation after retrieving the conductor cable fromthe well.

The apparatus of the invention may be employed in either cased oruncased wells. Best results to date have been obtained with cased wells.Erratic results are sometimes obtained with uncased wells where theearth formation contains great variations in strength in the exposedzone which is intended to be fractured by the apparatus of theinvention. In such cases fracture may take place in some unintendedplane which is substantially weaker than the plane intended to befractured.

The specific embodiments of the fracture initiation apparatus of thepresent invention shown in the drawings and described above are merelyillustrative and are not to be construed as limiting the scope of theinvention or the appended claims inasmuch as numerous modifications ofthe apparatus within the ambit of the invention will be readily apparentto those skilled in the art.

What is claimed is:

1. A fracture initiation gun assembly adapted to be lowered into a wellbore hole which assembly comprises a charge container and a plurality ofshaped explosive charges; said charge container being provided with aplurality of cylindrical charge chambers arranged in two straightparallel rows in a single plane, the axes of adjacent charge chambers ineach of said rows being substantially parallel, a plurality of fusepassages, one of said fuse passages being disposed between the adjacentends of each pair of charge chambers in the charge container consistingof one chamber from each of said parallel rows, and a plurality of airspaces between adjacent charge chambers in each of said rows ofchambers, said air spaces having a smaller cross section than saidcharge chambers, said cylindrical charge chambers having suflicientdepth to contain a shaped charge at the inner end thereof and provide aremaining stand-off space between the face of said shaped charge in thecharge chamber and the end of the charge chamber at the periphery ofsaid charge container, said shaped charges disposed in one row of chargechambers being faced to fire in one direction while those in the otherrow of charge chambers are faced to fire in the opposite direction andsaid fuses being adapted to fire said charges simultaneously.

2. A fracture initiation gun assembly adapted to be lowered into a wellbore hole which assembly comprises a cylindrical charge container andsix shaped explosive charges; said charge container being provided withsix cylindrical charge chambers arranged in two straight parallel rowsof three chambers each in a single plane with the axes of adjacentchambers in each of said rows parallel, three fuse passages, one of saidfuse passages being disposed between the adjacent ends of each of thethree pairs of charge chambers in the charge container consisting of onechamber from each of said parallel rows, and four air spaces one ofwhich is disposed between each pair of adjacent charge chambers in eachof said rows of chambers, said air spaces having a smaller cross sectionthan said charge chambers, said cylindrical charge chambers havingsufficient depth to contain a shaped charge at the inner end thereof andprovide a remaining stand-01f space between the face of said shapedcharge in the charge chamber and the end of the charge chamber at theperiphery of said charge container, the shaped charges disposed in onerow of charge chambers being faced to fire in one direction while thosein the other row of charge chambers are faced to fire in the oppositedirection and said fuses being adapted to fire said chargessimultaneously.

3. A fracture initiation gun assembly adapted to be lowered into a wellbore hole which assembly comprises a charge container and a plurality ofshaped explosive charges, said charge container being provided with twocharge blocks each containing three charge chambers, said charge blocksbeing disposed back to back in said charge container with the chargechambers of said blocks facing in opposite directions, the axes of theterminal charge chambers in each of said charge blocks making an angleof 8 with the axis of the central change chamber in said block, saidshaped explosive charges being disposed in said charge chambers with theshaped charges in one charge block faced to fire in one direction andthe charges in the other charge block faced to fire in the oppositedirection and a stand-off space between the shaped charges and theperimeter-of the main charge container.

4. An apparatus for initiating a fracture in a predetermined plane in anadjacent earth formation comprising a main charge container, a pluralityof shaped explosive charges and means to fire said chargessimultaneously; said main charge container containing two charge blocksdisposed therein in back-to back relationship, each of said chargeblocks defining at least two charge chambers for holding said explosivecharges, said explosive charges be ing arranged in the same plane andspaced apart substantially parallelwise at intervals such that the ratioof charge diameter to distance between charges is about 6.45.

5. An apparatus for initiating a fracture in. a horizontal plane in anadjacent earth formation comprising a main charge container, a pluralityof shaped explosive charges, and means to fire said chargessimultaneously, said main charge container containing two charge blocks,said two charge blocks being disposed within said container inbackto-back parallel relationship, each of said change blocks comprisinga plurality of charge chambers for holding said shaped explosivecharges, said charge chambers being arranged in a single plane andoriented to lie with their axes in a plane at right angles to thevertical axis of said main charge container, said shaped explosivespaced apart substantially parallelwise at intervals equal to about-1.1-5 inches between char-ge centers, said backto-back relation ofcharge, chambers, further defining a stand-off space before the innerwall of said main charge container.

' 6. An apparatus as in claim 5 wherein the end change chambers on eachside of the central charge chamber are oriented with an angular toe-inof 12 toward said central charge chamber.

Mims Apr. 27, 1926' 12 Malskat et al. Jan; 10, Kaltenberger May 9,Sweetman Feb. 26,. Sweetman Feb. 23, Church et a1. July 6, Bryant etca1. Feb. 7, 'Ifurechek Apr. 24, Church et a1. Aug. 7, Klot-z Jan. 29,Church at all June 24, Stewart July 15, Forsyth June 23,

Borins et a1. Aug. 4,

3. A FRACTURE INITIATION GUN ASSEMBLY ADAPTED TO BE LOWERED INTO A WELLBORE HOLE WHICH ASSEMBLY COMPRISES A CHARGE CONTAINER AND A PLURALITY OFSHAPED EXPLOSIVE CHARGES, SAID CHARGE CONTAINER BEING PROVIDED WITH TWOCHARGE BLOCKS EACH CONTAINING THREE CHARGE CHAMBERS, SAID CHARGE BLOCKSBEING DISPOSED BACK TO BACK IN SAID CHARGE CONTAINER WITH THE CHARGECHAMBERS OF SAID BLOCKS FACING IN OPPOSITE DIRECTIONS, THE AXES OF THETERMINAL CHARGE CHAMBERS IN EACH OF SAID CHARGE BLOCKS MAKING AN ANGLEOF 8* WITH THE AXIS OF THE CENTRAL CHARGE CHAMBER IN SAID BLOCK, SAIDSHAPED EXPLOSIVE CHARGES BEING DISPOSED IN SAID CHARGE CHAMBERS WITH THESHAPED CHARGES IN ONE CHARGE BLOCK FACED TO FIRE IN ONE DIRECTION ANDTHE CHARGES IN THE OTHER CHARGE BLOCK FACED TO FIRE IN THE OPPOSITEDIRECTION AND A STAND-OFF SPACE BETWEEN THE SHAPED CHARGES AND THEPERIMETER OF THE MAIN CHARGE CONTAINER.